Synthetic plastics materials have long been used for the packaging of foods and other materials which need protection from handling and from moisture. In recent years, it has become appreciated that, in addition, many foods and other sensitive materials benefit from being protected from atmospheric oxygen. However, many foodstuffs are packaged in modified atmospheres which are rich in carbon dioxide which acts as a biocide. Therefore, it is also important to reduce the rate of diffusion of the carbon dioxide from the package so that the quality of the packaged good is maintained. Furthermore, carbon dioxide barrier coatings could be used to preserve the quality of carbonated drinks bottled in plastic packaging by reducing the rate of decarbonation of the drink.
A wide variety of multilayer laminate structures have been developed to provide barrier properties and other performance characteristics suited to a pack's purpose. These laminates may be any combination of plastic, metal or cellulosic substrates, and may include one or more coating or adhesive layers. Laminates which include polymeric films having metals or inorganic compounds, such as silicon oxides, deposited thereon have been found to give good general barrier properties and are widely used. Moreover, the inorganic layer of these types of laminate is rather brittle and may crack or break when the laminate is flexed, resulting in a loss of the gas barrier properties.
A number of technologies exist which provide a barrier to carbon dioxide diffusion when applied to plastic or other substrates. DE 3518875A1, JP59054557A, and P. Dehassus in ‘Modern Plastics’ (1983), 60 (1), 86-88 all describe how PVDC coatings can be applied to PET bottles, intended for carbonated drinks, to reduce the rate of carbon dioxide diffusion. In the latter reference a reduction in CO2 migration from 0.89 cm3/day to 0.34 cm3/day was reported for a coated bottle.
The use of vapour deposition techniques to apply silicon oxide, aluminium oxide and aluminium layers to film surfaces is well known and excellent barriers to a range of gases, including CO2 are possible. R. Davis, in Annual Technical Conference Proceedings—Society of Vacuum Coaters (1998), 41st. 505-506 reported that the CO2 and other gas barrier of PET films coated with such layers was improved by a factor of approximately 10 times. Other instances of vapour deposited coatings providing barrier to carbon dioxide include WO2010065966A2 (atomic layer deposition method), C. Birchler; Fraunhofer-Institut fur Lebensmitteltechnologie and Verpackung, Munich, Germany. Coating (1994), 27 (8), 274-280, EP470777.
The use of lithium and potassium copolysilicate based coatings in providing CO2 barrier when applied to plastic films has also been reported in U.S. Pat. No. 5,882,798A.
E. Palasset in Fr. Caoutchoucs & Plastiques (2000), 77 (784), 42-43, describes how PET bottles can be coated with a composition containing an epoxy resin to improve their CO2 barrier properties. WO 95/26997A1 describes how beverage bottles can have their CO2 barrier improved by the application of coatings containing an epoxy-amine adduct. The use of acrylic polymers in providing CO2 barriers has also been reported in CN10160823A.
Instances of PVA (polyvinyl alcohol—also referred to as PVOH) and EVOH (ethylene-vinyl alcohol copolymer) being used to provide barrier to CO2 include GB2337470A and WO2009070800A1. In the latter reference a solution of PVA was blended with water-dispersible acrylate monomers and a UV photoinitiator. After application to a PET bottle the composition was UV-cured to provide not only improved CO2 barrier, but also improved water resistance to the coating.
Also worth noting here is the use of sol-gel type compositions comprising solutions of PVA/EVOH and hydrolysed alkoxy-silanes applied to the surface of the inorganic layer (silicon oxide, aluminium oxide or aluminium) of a pre-coated film to further improve the gas barrier performance of the coated film. These coatings not only enhance the barrier performance of the inorganic layer but also provide a degree of protection during printing and lamination. Layers of the inorganic coatings are very fragile and have poor flex resistance. However, including these sol-gel coatings confers a degree of improved barrier performance after these types of laminates have been flexed or folded. Specific references reporting the use of such protective coatings in improving the CO2 barrier performance of coated films include JP2008080540A. The use of coatings containing PVA/EVOH and hydrolysed alkoxy-silanes applied to plastic films having no pre-existing inorganic barrier layer, and providing a CO2 barrier, has also been reported by M. Minelli in Polym. Eng. Sci. (2010), 50 (1), 144-153, where it was reported that the CO2 transmission rates through films such as PET and OPP could be reduced by over 100 times.
More recently, oxygen gas barrier coatings comprising dispersed clay, especially nanoparticles, and a hydrophilic polymer, such as polyvinyl alcohol (PVA) or ethylene-vinyl alcohol copolymer (EVOH), have been used, as described, for example, in U.S. Pat. No. 6,599,622, EP 0 590 263, JP01313536A2, JP2007-136984, EP 0 479 031, U.S. Pat. No. 4,818,782, WO 2009/027648A1 and WO 2009/098463A1.
JP 11-246729 discloses a resin composition containing polyvinyl alcohol, a water-soluble polyacrylic acid system compound, and an inorganic laminar compound for use as a gas barrier coating. U.S. Pat. No. 6,709,735 B2/EP 1 451 008 B1 and U.S. Pat. No. 6,991,837 B2 similarly disclose the use of compositions of PVOH and copolymers of acrylic acid and maleic acid with a molecular weight of from about 3500 to about 5000 to prepare oxygen barrier coatings.